It may be worth writing a few scenes chapters to acquaint yourself with the 'waking up in a new world' experience of the character, but not keeping it in the story. Find the moment when he can't stand still and can't emote and can't move forward without a decision to get, do, become. Make that decision, with the accompany tension, the opening of the story. What led up to that point then becomes backstory to filter into the story as it becomes necessary (but not at the opening).
Start with a problem that has to be solved with an intention to act, followed by the action and problems that actions causes.
By action, I don't mean big action like car (or horse) chases, fisticuffs, etc. What I mean is the character acting on their own decisions to do something. 'I'll go to the top of the hill and see where the river leads. If it leads to [something I recognise], I'll follow the river.'
Then the obstacles create difficulties with reaching that goal. Climbing the hill is much harder than it looked, much higher, rocky/snowy/wet, animals want to eat him, etc. but he struggles through and reaches the top and the trees block his view so he climbs the tallest tree and looks out and sees ... nothing he recognises.
The above is a simple exercise to see progression through decision and action. The character makes a decision to get, do, become, and then acts on the decision. It's also how I, as a reader, learn about the character through what he does (agency), how he does it, and why he does it. I get to know him through his purpose and determination, his needs and fears, his goals and struggles.
